Java ClassLoader and Usage of Custom ClassLoader

The Java Classloader is a part of Java Runtime Environment that dynamically loads Java classes into the Java Virtual Machine. Usually classes are only loaded on demand(lazy loading).i.e. There are exactly two cases: When the first instance of the class is created (MyClass cl=new MyClass()) and static reference to a class is made (System.out.).

The class loader is responsible for locating libraries, reading their contents, and loading the classes contained within the libraries. This loading is typically done "on demand", in that it does not occur until the class is actually used by the program. There is a few difference to get the first class running (which is why you have to declare the main() method as static, taking a string array as an argument), but once that class is running, future attempts at loading classes are done by the class loader.

A class with a given name can only be loaded once by a given classloader(Note: possible with custom class loader .Will discuss later) .

Each Java class must be loaded by a class loader. Furthermore, Java programs may make use of external libraries (that is, libraries written and provided by someone other than the author of the program) or they may be composed, at least in part, of a number of libraries.

When the JVM is started, three class loaders are used :

1. Bootstrap class loader
2. Extensions class loader
3. System class loader
   

This embedded loaders are called the primordial class loaders.The class loaders are implemented using java.lang.ClassLoader abstract class.

The bootstrap class loader loads the core Java libraries (<JAVA_HOME>/lib directory). This class loader, which is part of the core JVM, is written in native code.

The extensions class loader loads the code in the extensions directories (<JAVA_HOME>/lib/ext or any other directory specified by the java.ext.dirs system property). It is implemented by the sun.misc.Launcher$ExtClassLoader class.

The system class loader loads code found on java.class.path, which maps to the system CLASSPATH variable. This is implemented by the sun.misc.Launcher$AppClassLoader class.

Delegation model

The ClassLoader class uses a delegation model to search for classes and resources. Each instance of ClassLoader has an associated parent class loader. When requested to find a class or resource, a ClassLoader instance will delegate the search for the class or resource to its parent class loader before attempting to find the class or resource itself. The virtual machine's built-in class loader, called the "bootstrap class loader", does not itself have a parent but may serve as the parent of a ClassLoader instance.

 The class loaders in Java are organized in a tree. By request a class loader determines if the class has already been loaded in the past, looking up in its own cache. If the class is present in the cache the CL returns the class, if not, it delegates the request to the parent. If the parent is not set (is Null) or can not load the class and throws a ClassNotFoundException the classloader tries to load the class itself and searches its own path for the class file. If the class can be loaded it is returned, otherwise a ClassNotFoundException is thrown. The cache lookup goes on recursively from child to parent, until the tree root is reached or a class is found in cache. If the root is reached the class loaders try to load the class and unfold the recursion from parent to child. Summarizing that we have following order:

• Cache
• Parent
• Self
  

This mechanism ensures that classes tending to be loaded by class loaders nearest to the root. Remember, that parent class loader is always has the opportunity to load a class first. It is important to ensure that core Java classes are loaded by the bootstrap loader, which guarantees that the correct versions of classes such as java.lang.Object are loaded. Furthermore it ensures, that one class loader sees only classes loaded by itself or its parent (or further ancestors) and it cannot see classes loaded by its children.

The picture illustrates the hierarchy of class loaders. Root loader is bootstrap class loader which has native implementation and cannot be instantiated by Java code.

It is followed by extension class loader, which is primary responsibility to load classes from the extension directories and provides ability to simply drop in new JVM extensions, without requiring modification to the user’s classpath. The system or application class loader responsible for loading classes from the path specified by the CLASSPATH environment variable. This class loader will be returned by the ClassLoader.getSystemClassLoader() method.

Why write a Custom ClassLoader?

If the JVM has a ClassLoader, then why would you want to write another one? Good question. The default ClassLoader only knows how to load class files from the local filesystem. This is fine for regular situations, when you have your Java program fully compiled and waiting on your computer.

But one of the most innovative things about the Java language is that it makes it easy for the JVM to get classes from places other than the local hard drive or network. For example, browsers use a custom ClassLoader to load executable content from a Web site.

There are many other ways to get class files. Besides simply loading files from the local disk or from a network, you can use a custom ClassLoader to:

• Automatically verify a digital signature before executing untrusted code
• Transparently decrypt code with a user-supplied password
• Create dynamically built classes customized to the user's specific needs
• to load or unload classes at runtime (for example to load libraries dynamically at runtime, even from a HTTP resource). This is an important feature for:
  • implementing scripting languages,
  • using bean builders,
  • allowing user-defined extensibility
  • allowing multiple namespaces to communicate. This is one of the foundations of CORBA / RMI protocols, for example.
• to change the way the bytecode is loaded (for example, it is possible to use encrypted Java class bytecode).
• to modify the loaded bytecode (for example, for load-time weaving of aspects when using aspect-oriented programming).
 

Please visit Custom ClassLoader Example -FileSystemClassLoader to study more about custom class loader.